1. Technical Field
The present invention relates to an image reading apparatus and an image reading system for reading an image from an original document.
2. Related Art
An image reading apparatus that scans an image from an original document is well-known in the prior art. Such an image reading apparatus is provided with (a) photoelectric conversion elements (i.e. photoelectric transducers) that receive a light reflected from or transmitted through an original document so as to accumulate electric charge, the amount of which depends on the amount of the light received, while the photoelectric conversion elements move in a predetermined direction with respect to the original document, and (b) an encoder that detects the vector amount of the movement in the predetermined direction so as to output a detection signal for each relative motion vector amount that corresponds to the size of a pixel that is to be scanned.
In such a configuration, one cycle of operation includes a light reception operation, in which the photoelectric conversion element receives a light coming from the original document in a predetermined color order (for example, in the order of R (red), G (green), B (Blue)) for a predetermined light reception time duration where the light reception operation is executed once for each color component, and an electric charge takeout operation, in which electric charge is taken out of the photoelectric conversion element after each light reception operation executed for the light reception time duration (hereafter referred to as electric charge discharge operation). According to such a configuration, image scanning is executed by repeating the cycle of operations at each time when the detection signal is outputted from the encoder (for example, refer to JP-A-11-55471).
In some of the image reading apparatuses, the above-described light reception operation and the electric charge discharge operation are carried out based on a detection signal EP, which is outputted from the encoder, and the clock pulses CP, which are outputted at intervals of a predetermined time t using the detection signal EP as a starting point thereof (this operation is hereafter referred to as a second control).
FIG. 17A is an explanatory diagram that schematically illustrates an example of the operation timing of the light reception operation and the electric charge discharge operation when the second control is adopted. As illustrated in FIG. 17A, three of clock pulses, that is, a first clock pulse CP (#1), a second clock pulse CP (#2), and a third clock pulse CP (#3), are outputted for each cycle at the interval of a predetermined time “t” using the output of the detection signal EP from the encoder as a starting point thereof. More specifically, the first clock pulse CP (#1) is outputted in synchronization with the start of the detection signal EP; the second clock pulse CP (#2) is outputted after a lapse of the predetermined time t after the output of the first clock pulse CP (#1); and the third clock pulse CP (#3) is outputted after a lapse of the predetermined time t after the output of the second clock pulse CP (#2). It should be noted that in the figure, the time interval between the third clock pulse CP (#3) in a certain cycle and the first clock pulse CP (#1) in the next cycle is denoted as t′ rather than t because the output timing of the first clock pulse CP (#1) is subjected to variation depending on a possible delay in the output of the detection signal EP. In other words, if it is assumed that there is not any delay in the outputting of the detection signal EP, which is an ideal condition, the above time interval t′ is equal to the predetermined time t, where the figure illustrates such an ideal condition.
According to the second control, triggered by the above-described first clock pulse CP (#1), which is outputted at the timing of the detection signal EP outputted from the encoder, the light reception operation is performed for the first color component among color components R, G, and B (for example, R color component). Through the light reception operation, electric charge is accumulated in the photoelectric conversion elements, where the amount of the electric charge accumulated therein depends on the amount of the light received during a light reception time duration T. Then, the accumulated electric charge is taken out of the photoelectric conversion elements as the electric charge of the first color component at the timing of the second clock pulse CP (#2).
Triggered by the second clock pulse (#2), the light reception operation for the second color component (for example, G color component) is performed so that electric charge is accumulated in the photoelectric conversion elements, where the amount of the electric charge accumulated therein depends on the amount of the light received during the light reception time duration T. Then, the accumulated electric charge is taken out of the photoelectric conversion elements as the electric charge of the second color component at the timing of the third clock pulse CP (#3).
Triggered by the third clock pulse (#3), the light reception operation for the third color component (for example, B color component) is performed so that electric charge is accumulated in the photoelectric conversion elements, where the amount of the electric charge accumulated therein approximately depends on the amount of the light received during the light reception time duration T. Then, the accumulated electric charge is taken out of the photoelectric conversion elements as the electric charge of the third color component at the timing of the first clock pulse CP (#1), which is outputted at the timing of the next detection signal EP outputted from the encoder. In this way, electric charge for each of the color components R, G, and B is taken out, which means that the scanning of an image that constitutes one pixel is completed in the predetermined direction.
However, according to the second control described above, there is a problem in that the amount of electric charge actually accumulated for the third color component (for example, the Blue color component) tends to be larger than the amount of electric charge that is supposed to be accumulated for this color component during the light reception time duration T described above, which could cause degradation in the quality of the scanned image.
The reason why such a problem occurs is that, when there is some variation (i.e. scatter, dispersion) in the movement speedin the predetermined direction, as illustrated in FIG. 17B, the time interval from the end of the light reception time duration T for the third color component (e.g. B component) till the outputting of the detection signal EP from the encoder is made longer than the ideal condition illustrated in FIG. 17A because of such a variation by the length of a delay time td. In other words, the electric charge could still be accumulated in the photoelectric conversion elements due to the presence of a dark current, etc., even though no light reception operation is performed after the end of the light reception time duration T; and if such a phenomenon occurs so that the timing of the detection signal outputted from the encoder is delayed by the delay time td, electric charge accumulated for the third color component (e.g. B component) could contain any unwanted additional electric charge, making the accumulated amount of the electric charge thereof larger than the amount of electric charge that is supposed to be accumulated for this color component. Such an additional electric charge accumulated for the third color component (e.g. B component) has a direct and adverse effect on the precision of image data generated based on the accumulated electric charge, which results in degradation in the quality of the scanned image.
As a first control for preventing image quality from being degraded, the following configuration could be adopted. FIG. 17C is an explanatory diagram that schematically illustrates an example of the operation timing of the light reception operation and the electric charge discharge operation when the first control is adopted. Herein, the major difference between the first control and the second control lies in that, in contrast to the second control where three of clock pulses are outputted for each output of the detection signal EP from the encoder, four clock pulses, that is, the first clock pulse CP (#1), the second clock pulse CP (#2), the third clock pulse CP (#3), and the fourth clock pulse CP (#4), are outputted in the first control, and in addition thereto, a discarding processing is performed in the first control so as to discard any unwanted additional electric charge accumulated due to the presence of a dark current after the electric charge discharge operation for the third color component (e.g. B component) that is performed at the timing of the fourth clock pulse CP (#4).
More specifically, as illustrated in FIG. 17C, in the same manner as the second control, the first control works as follows: the light reception operation for the first color component (e.g. R component) is performed based on the first clock pulse CP (#1); the electric charge discharge operation for the first color component as well as the light reception operation for the second color component (e.g. G component) is performed based on the second clock pulse CP (#2); and the electric charge discharge operation for the second color component as well as the light reception operation for the third color component (e.g. B component) is performed based on the third clock pulse CP (#3). After receiving the light of the third color component for the light reception time duration, the photoelectric conversion elements hold electric charge accumulated therein, the amount of which depends on the amount of the light received during the light reception time duration.
However, in the second control, the electric charge accumulated for the third color component (e.g. B component) is not taken out of the photoelectric conversion elements at the timing of the next detection signal EP outputted from the encoder as in the first control, but based on the fourth clock pulse CP (#4).
In the second control, the electric charge to be discarded is taken out of the photoelectric conversion elements at the output timing of the next detection signal EP. That is, in the second control, any unwanted additional electric charge accumulated due to the presence of a dark current after the electric charge discharge operation for the third color component but before the outputting of the next detection signal EP is discarded through the electric discharge operation executed at the output timing of the next detection signal EP. Therefore, even if there occurs a delay in the output of the detection signal EP, which poses a problem in the second control, it is possible to avoid or reduce degradation in the image quality due to any unwanted additional electric charge, which is achieved by the discarding processing described above.
However, the first control has a disadvantage in that its scanning speed is slower than that of the second control because the former takes extra time for the discarding of electric charge. That is, as a comparison between FIG. 17A and FIG. 17C shows clearly, the first control illustrated in FIG. 17C requires extra time for scanning than the second control illustrated in FIG. 17A does by the length of an additional predetermined time t taken for generation of the fourth clock pulse CP (#4). In addition, depending on requirements of each situation, the user can change his preference as to which one of the reading speed and the image quality should be given a higher priority. Accordingly, the user-friendliness is enhanced because the user is allowed to select between the first control and the second control depending on the requirements of each situation.